1. Field of the Invention
The present invention relates to a production method for an optical disc master which uses an inorganic resist. The present invention also relates to an optical disc master, a stamper, and an optical disc obtained as a result of the production method.
2. Description of the Related Art
Various formats of optical discs for various uses such as CD and DVD formats are proposed in the related art. Optical disc substrates for use in any format are generally formed by injection molding of a resin material. A concave-convex pattern including pits and grooves is formed on the surface of an optical disc substrate.
The concave-convex pattern formed on the optical disc substrate represents an information signal. An optical information storage medium with a high capacity is obtainable by miniaturizing and densifying the concave-convex pattern.
Such a concave-convex pattern formed on an optical disc substrate is formed by transferring a concave-convex pattern formed on an optical disc master to the optical disc substrate. A common method for forming an optical disc master for use in the production of DVDs is described below with reference to FIGS. 6 and 7.
First, as shown in FIG. 6A, a glass substrate 101 with a sufficiently smoothed surface is prepared. Then, as shown in FIG. 6B, a photosensitive organic resist layer 102 is formed on the glass substrate 101. The organic resist layer 102 is formed by placing the glass substrate 101 on a rotary table (not shown) and applying an organic resist material to the entire surface of the glass substrate 101 while rotating the rotary base at a predetermined rotational speed.
Then, as shown in FIG. 6C, the organic resist layer 102 is exposed to an exposure laser beam 103 in a desired pattern including pits and grooves. A portion of the organic resist layer 102 that corresponds to the spot diameter of the laser beam 103 is exposed through a photochemical reaction induced by the laser beam 103.
Then, the resulting organic resist layer 102 is developed using a developing solution, and an exposed portion or an unexposed portion of the organic resist layer 102 is removed using a chemical solution. In this way, as shown in FIG. 6D, an optical disc master 110 with the organic resist layer 102 formed in a predetermined concave-convex pattern on the glass substrate 101 is obtained.
The minimum pit length (spot diameter) P that may be achieved with the photosensitive organic resist layer 102 in the exposure and development steps discussed above is determined by the following formula:P=K×λ/NA 
In the formula, λ represents the wavelength of the laser beam 103 used for exposure, NA represents the numerical aperture of an objective lens for converging a luminous flux emitted from a light source on the organic resist layer 102, and K represents a proportionality constant determined by the combination of the wavelength of the laser beam 103 used and the material of the organic resist layer 102, which generally may be about 0.5 to 0.8. If it is assumed in the example that K is 0.87, the wavelength λ of the laser beam 103 used is 413 nm, and NA is 0.9, the minimum pit length (spot diameter) P is about 0.87×413/0.9=399 nm.
Then, as shown in FIG. 7E, a metallic nickel film 104 is deposited over the surface of the concave-convex pattern of the optical disc master 110 by electroforming. Then, the metallic nickel film 104 is peeled from the optical disc master 110 and undergoes a predetermined process, completing a stamper 104a shown in FIG. 7F.
With the thus formed stamper 104a disposed in a cavity of an injection molding apparatus, injection molding is performed using polycarbonate which is a thermosetting resin as shown in FIG. 7G. Consequently, as shown in FIG. 7H, an optical disc substrate 105 to which the concave-convex pattern formed on the stamper 104a has been transferred is formed.
An optical disc master on which a series of pits with a minimum pit length of 0.4 μm and a track pitch of 0.74 μm are formed is formed by the method for forming an optical disc master. The optical disc master 110 is used to produce read-only DVDs (DVD-ROMs), for example. DVDs produced using the thus configured optical disc master 110 have an information storage capacity of 4.7 GB for one side of an optical disc with a diameter of 12 cm.
In a concave-convex pattern on an optical disc master formed through a photochemical reaction using an organic resist layer, however, side surfaces of grooves in the concave-convex pattern may be widened more than desired or may not be formed to be smooth. In the case where the concave-convex pattern includes a sharply bent groove, the corner of the sharply bent portion may be formed not in a predetermined shape but to be more or less rounded. If the concave-convex pattern is not formed in a desired shape, a sufficient margin may not be secured, for example, in the case of densifying the concave-convex pattern to fabricate a high-capacity optical disc.
In recent years, high-density optical discs conforming to a Blu-ray Disc (registered trademark, hereinafter referred to as “BD”) standard have been becoming more common. An optical disc conforming to the BD standard has a storage capacity of approximately 25 GB for a single side of a single-layered disc and a storage capacity of approximately 50 GB for a single side of a double-layered disc. In order for a single side of an optical disc with a diameter of 12 cm to have an information storage capacity of 25 GB, it is necessary that the minimum pit length and the track pitch be reduced to about 0.17 μm and 0.32 μm, respectively.
With this in view, in order to form an optical disc master for BDs by the method for forming an optical disc master according to the related art discussed above, it is necessary to use a light source that emits a laser beam with a wavelength λ of 180 nm in the case where the proportionality constant K is fixed and NA is 0.95, for example. That is, an exposure apparatus for forming an optical disc master for DVDs may not be used as it is. The wavelength of 180 nm is shorter than the wavelength of an ArF laser, 193 nm. The ArF laser is being developed as a light source for next-generation semiconductor lithography. An exposure apparatus that uses such a short-wavelength beam employs special optical components such as lenses besides the laser-emitting light source and may be very expensive.
Japanese Unexamined Patent Application Publication No. 2006-185529 (JP-A-2006-185529) discloses an exposure apparatus for both forming a high-density optical disc master conforming to the BD standard and forming a low-density optical disc master for DVDs, for example, which have a storage density lower than that conforming to the BD standard. In the exposure apparatus according to JP-A-2006-185529, a spot-forming optical element is disposed in an optical path of an exposure laser beam to irradiate a resist layer with the laser beam with a desired spot diameter for exposure of the resist layer. Adjusting the spot diameter allows the same exposure apparatus to form both an optical disc master for BDs and an optical disc master for DVDs.
In the exposure apparatus according to JP-A-2006-185529, however, it is necessary to provide a spot-forming optical element in the optical path of the exposure laser beam of the exposure apparatus. Moreover, an operation for varying the spot diameter is troublesome.